text:
Is it safe to fly ultralights at 6-7 weeks pregnancy?
Are there regulations or official guidance regarding pregnant women flying ultralight aircraft at 6-7 weeks pregnancy (amenorrhea weeks)?

I would get an OK from the doctor first, while in a perfect world there is probably nothing wrong with this, it depends greatly on the woman and if it is a high risk pregnancy or not.

See the answers here for some related information

@Pondlife Well, until your edit, it was an individual question ;)

If you are asking for regulations or official guidance, can you please add which country / legislation you are in?

@Pondlife My question was more specific (ultralights aren't "airline" aircraft, and I was afraid that the vibrations might be a bigger problem). The medical corps vetted the flight though, so all is good. Thanks all!

Ultimately the decision of whether or not a pregnant woman should be piloting an aircraft is between her and her doctor, and you shouldn't take the commentary of strangers on the internet as medical advice ("I am not a doctor, and even if I were I am not your doctor").
That said while I'm not sure what EASA has to say on the subject I know the FAA does not consider pregnancy to be a "disqualifying medical condition" for operations requiring a medical certificate, so it would logically not be disqualifying for operations which do not require a medical certificate (gliders, balloons, or ultralights) either.

Ultimately much of the advice out there comes down to "you can fly through the second trimester, and shouldn't fly in the third trimester" - assuming a normal and uncomplicated pregnancy, with the arguments against flying in the third trimester largely focusing on the onset of labor, obstruction of the flight controls, or difficulty with the aircraft's restraints (seatbelts).
Some specific concerns that have been identified for pregnant pilots include:

Inability to get full range of movement on the flight controls
If the pilot's "baby bump" is large enough to obstruct the free movement of the flight controls they probably shouldn't be flying an airplane.
This is a concern for all pilots with larger-than-average abdomens, but as pregnant women tend to enlarge quickly it's something they need to be aware of on each flight.
Onset of labor
All the women I know who have given birth have lead me to believe it would be rather difficult to simultaneously fly an aircraft and give birth, so it would be sensible to avoid single-pilot operations (including ultralight flying) late in pregnancy where this might be an issue.
Blood Clots
Sitting in the same position for long periods of time increase the chance of blood clots. This is a problem for all pilots, but pregnant women can be more susceptible to blood clots so flights should be kept short if possible.
Hypoxia
Mom is breathing for two, and physiologically the fetus has a priority claim on oxygenated blood. As a result pregnant women may require oxygen at lower altitudes, but this is not likely to be a concern for ultralight pilots as you won't be climbing that high.

Piloting an ultralight probably carries a much higher risk of physical injury to the baby than a commercial two-pilot flight would.  A rough landing in a 737 is probably going to be more gentle than a rough landing in a motorized three-wheeled hang glider.

There's nothing in regulations specifically about pregnancy, a pregnant woman can fly an airplane providing she is compliant with FAR 61.53, meaning that she would voluntarily "self ground" if she was unsure of her ability to fly.

Subpart A General Sec. 61.53 Prohibition on operations during medical
  deficiency.
(a) Operations that require a medical certificate. Except as provided
  for in paragraph (b) of this section, a person who holds a current
  medical certificate issued under part 67 of this chapter shall not act
  as pilot in command, or in any other capacity as a required pilot
  flight crewmember, while that person:
(1) Knows or has reason to know of any medical condition that would
  make the person unable to meet the requirements for the medical
  certificate necessary for the pilot operation; or
(2) Is taking medication or receiving other treatment for a medical
  condition that results in the person being unable to meet the
  requirements for the medical certificate necessary for the pilot
  operation.
(b) Operations that do not require a medical certificate. For
  operations provided for in Sec. 61.23(b) of this part, a person shall
  not act as pilot in command, or in any other capacity as a required
  pilot flight crewmember, while that person knows or has reason to know
  of any medical condition that would make the person unable to operate
  the aircraft in a safe manner.
(c) Operations requiring a medical certificate or a U.S. driver's
  license. For operations provided for in Sec. 61.23(c), a person must
  meet the provisions of--
(1) Paragraph (a) of this section if that person holds a valid medical
  certificate issued under part 67 of this chapter and does not hold a
  current and valid U.S. driver's license.
(2) Paragraph (b) of this section if that person holds a current and
  valid U.S. driver's license.

Pregnancy impacts women in different ways, and no two pregnancies are the same. At 6-7 weeks a woman would barely be showing, so control reach would not be a problem. The real issues come from the hormonal changes which can cause a variety of effects at that point. In rare cases women have not even realized they are pregnant at 6-7 weeks or even later, other women would be retching their guts out for almost their whole term.
Every pilot has to determine their medical fitness to fly every time the are going to sit in the cockpit, pregnancy is just one factor among many.


text:
Is it legal to fly an aircraft with no navigation beacons, and only solid lighting?
Is it permissible for an aircraft to fly without red, green, and white strobe position lights?
Is it possible for any aircraft to legally fly with only a solid white or solid red light on the underbelly?

The use of the words "navigation beacons" is going to cause confusion. I think you are referring to the position lights. If so, I'd clarify that language.

Secret goverment/military aircraft.

All gliders fly without position lights. So the answer must be Yes, even if no external light is fitted.

Yes, during the day. Lights are required only for night or IMC flying, and a lot of ultralights and LSAs intended for day VFR only therefore do not bother.

I recently witnessed a V type lighting configuration of one solid red, and two solid white lights.  It was night so I'm unsure if it was 1 or 3 aircraft, but my gut tells me it was 3 flying in formation.

This answer assumes that the question is in the context of the FAA's jurisdiction.
The red, green and white lights are position lights. They are required from sunset to sunrise by 14 CFR 91.203.

(a) During the period from sunset to sunrise (or, in Alaska, during the period a prominent unlighted object cannot be seen from a distance of 3 statute miles or the sun is more than 6 degrees below the horizon)—
...
(1) Operate an aircraft unless it has lighted position lights;

A solid red or solid white light on the underbelly doesn't correspond to a configuration that I am familiar with and doesn't meet the legal requirements.
I have, however, seen some older varieties of rotating beacons that if broken may become a solid red light mounted on the underbelly. Some older models actually have a motor that actuates rotation. If one had a failed motor it could appear as a solid red light.

The strobes are anticollision lights. The answer is also in 14 CFR 91.203 . If anticollision lights are installed, they must be used unless the pilot turns them off in the interest of safety. The most common reason for this is if the aircraft is flying in the clouds and the diffuse strobing lights are a distraction or cause of disorientation such as flicker vertigo.

(a) During the period from sunset to sunrise (or, in Alaska, during the period a prominent unlighted object cannot be seen from a distance of 3 statute miles or the sun is more than 6 degrees below the horizon)—
...
(b) Operate an aircraft that is equipped with an anticollision light system, unless it has lighted anticollision lights. However, the anticollision lights need not be lighted when the pilot-in-command determines that, because of operating conditions, it would be in the interest of safety to turn the lights off.

14 CFR 91.205 is also relevant. Parts of it deal with the FAA requirements for both position and anticollision light systems. It states that aircraft may not be operated without position lights. It also states that aircraft may not be operated without anticollision lights installed with some exceptions.
There are some provisions by which a flight may be conducted legally with broken lights so that it may be repaired. One such provision is also found in 14 CFR 91.205.

(11) For small civil airplanes certificated after March 11, 1996, in accordance with part 23 of this chapter, an approved aviation red or aviation white anticollision light system. In the event of failure of any light of the anticollision light system, operation of the aircraft may continue to a location where repairs or replacement can be made.

If you're really interested, and I mean really really interested, the FAA released AC 20-74 which contains a lot of technical data on exactly how these lights are to be built in terms of intensity, color and position. Delving into this document should be reserved for the truly bored and those who manufacture aircraft lighting.

I'm not sure if aircraft has any navigational beacon.
The red and white (strobe)  lights are anti collision lights.
According to Aeronautical Information Manual(AIM), 4−3−23. Use of Aircraft Lights,

An aircraft anti−collision light system can use one or more rotating beacons and/or strobe lights, be colored either red or white, and have different (higher than minimum) intensities when compared to other aircraft. Many aircraft have both a rotating beacon and a strobe light system.

Usually, the anti-collision/position lights are to be operated during night flying. From the same manual:

Aircraft position lights are required to be lighted on aircraft operated on the surface and in flight from sunset to sunrise. In addition, aircraft equipped with an anti−collision light system are required to operate that light system during all types of operations

However, the pilot can turn them off if there are safety concerns.

However, during any adverse meteorological conditions, the pilot−in−command may determine that the anti−collision lights should be turned off when their light output would constitute a hazard to safety

In your case, the pilot may have turned off the anti-collision lights and operated only the position lights.

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text:
Who said "A little of all of us goes with every pilot we lose"?
The only attribution I’ve ever seen for the quotation below is “Anonymous FAA Inspector.”

Whenever we talk about a pilot who has been killed in a flying accident, we should all keep one thing in mind. He called upon the sum of all his knowledge and made a judgment. He believed it so strongly that he knowingly bet his life on it. That his judgment was faulty was a tragedy, not stupidity. Every inspector, supervisor, and contemporary who ever spoke to him had an opportunity to influence his judgment, so a little of all of us goes with every pilot we lose.

Who originally said or wrote these words? What were the motivating circumstances?

I've seen it attributed in a lot of places, but they all say "anonymous" or "unknown", it may just be one of those anecdotal sayings that "somebody" said, that is really the combination of a number of different people and ideas combined into one.

Here is on to add to the list: "We must learn from the mistakes of others because we will not live long enough to make them all ourselves."

William Garvey in "Viewpoint"
The authoritative reference is in this article:
Aftermath: Lessons Learned  (Aviation Week)
Excerpt:

The last words of William Garvey's January Viewpoint -- ". . . so a
little of all of us goes with every pilot we lose." -- struck me
("Aftermath," page 7). I had just returned from the small town of
Vesoul, France, where I attended a ceremony marking the 40th
anniversary of the death of a Mirage III pilot. Lt. Patrick de
Barbeyrac Saint-Maurice died while flying an intercept training
mission near Vesoul on Nov. 3, 1966. The cause of the accident was
never fully determined, other than the ...
THIS CONTENT REQUIRES SUBSCRIPTION ACCESS
Paywall stops reading more of it.

What is the complete quotation?

Do not have enough reputation to comment, but will add this as a potential answer source:

— Captain A. G. Lamplugh, British Aviation Insurance Group, London. c. early 1930's. This famous phrase has been reproduced on posters and plaques many times, almost always with the attribution of 'anonymous.' I was told at a book signing that André Priester (one of the first Pan Am employee's) may have said it, and decided to check this with the late R. E. G. Davies, then curator of air transport history at the Smithsonian and author of a book on Pan Am. Ron called me back and told me the phrase pre-dates Priester. His research showed the originator of the phrase was Captain Lamplugh, who was quite well known in British aviation circles after WWI

Source: http://www.skygod.com/quotes/safety.html

The page you linked to gives that as the source for a different quote. The one the OP asked about is further down the same page, and listed as "author unknown".

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text:
What happens to incoming flights when the airport shuts down?
In Die Hard 2 terrorists take over an airport's ILS and landing lights on a snowy night (visual landing is impossible) and start making threats essentially holding the planes hostage.
The tower immediately sends incoming flights away to the diversion airports and "rack and stack" those already in the landing holding patterns. They also notify the pilots what happened mentioning that after the crisis is over they will start landing planes in order of fuel emergency and that radio may go dead later. Later in the movie the planes in the air actually start running out of fuel and one actually tries to land in the fog.
What is the protocol when you are a pilot in that situation? Do you just turn to your alternate when you have been circling around for 30 minutes (the "Final reserve fuel" for commercial jets).

I asked about the situation here. These comments can thus be purged if necessary.

If a "terrorist takeover" like this were to happen in the real world...
...and the tower still had communication capability:
The tower controllers would advise pilots of the situation, and likely instruct them to return to the Approach frequency to coordinate their next steps (probably a landing at an alternate airport).
Presumably the tower would still have telephone communication as well, and be able to let Approach know to expect the planes to come back because of the emergency, otherwise the pilots would certainly inform the approach controllers of what's going on when they switch back to that frequency.
...and the tower has no communication capability:
With no response from the tower the usual thing for pilots to do is return to the Approach frequency to find out what's going on. The pilots could elect to attempt a landing anyway, treating the airport as an untowered field (this somewhat famously happened at KDCA, for somewhat different reasons), or proceed to an alternate airport.
If the weather required an instrument approach and the ILS radio signals are not transmitting this would be something the pilots could detect pretty easily (similarly if they were turned off while flying the approach there would be indications in the cockpit that the signal was lost and the pilot would execute the missed-approach procedure), and they would proceed to an alternate airport, again coordinating with the approach controllers.
...in any case:
No matter what the situation the aircraft would divert to their alternate airports long before they were in danger of running out of fuel. They would notify ATC of the situation and proceed to an alternate airport. (If not cleared by ATC to proceed to an alternate at some point any sane pilot will declare an emergency for low fuel and proceed on their emergency authority to make a safe landing somewhere.)

So basically there's some "artistic liberties" taken in Die Hard 2 in the interest of making it a movie you'd want to sit through.
A movie where a bunch of terrorists take over an airport and a bunch of flights divert to alternate airports (mildly inconveniencing the passengers) while the National Guard surrounds the airport and deals with the terrorists wouldn't sell nearly as many tickets!

As a former Guardsman, don't count on the Guard. Probably SWAT, FBI, or Homeland Security. Most people don't live near their armory and are just regular citizens. Terrorist activity is criminal in nature, not necessarily war-like if say the Russians decided to invade. Sorry to hijack your answer :)

Out of interest, would capturing Washington Dulles actually cause a loss of ATC? I kind of feel like there ought to be someone else able to take control of such a significant airspace when the airport goes silent, and clear the pilots to proceed elsewhere. Adjacent airspaces? The region isn't short of metro areas. But I don't know why I feel that since I know nothing about ATC ;-)

@staticx Perfectly valid points :-) In the real world local police and/or the FBI would probably deal with the issue. Movie Land though you'll usually wind up with military or Guard intervention the second the "T" word is uttered, because police are boring :-)

@SteveJessop Capturing an airport would only affect services provided by that airport (the same way the controllers falling asleep only affected DCA). There's also a huge amount of redundancy in the system: If you can't raise the tower you could go one step up the chain and talk to the Approach (TRACON) controllers. If the TRACON and tower are colocated (or you can't get in touch with them for some other reason) you could try the center controllers, or flight service, or flight watch. Taking out all ATC services isn't impossible, but it's certainly impractical as an attack :)

There was an episode of Air Crash Investigation where a South American plane was in a holding pattern near New York for so long it ran out of fuel and crashed on Long Island (IIRC). So it has happened at least once!

one more impossibility from the movie (if I remember correctly) is that they had "changed the ILS to cause the aircraft to crash" so probably somehow redirected the ILS signal to send the aircraft somewhere different from the runway. That'd need physical access to the antenna arrays and a lot of time and tooling to change their setup.

@jwenting they move ground level 200 feet down or something like that using only a computer, either way at a 5 degree glideslope that would mean "touching down" half a mile short

@ratchetfreak rl that'd mean changing the physical position of the transmitters...

@sevenseacat: https://en.wikipedia.org/wiki/Avianca_Flight_52 . "NTSB determined that the crash occurred due to the flight crew failing to properly declare a fuel emergency"

Your link to the KDCA is broken, it returns a broken video page

As Pilot In Command, once you determine your intended destination is unavailable for any reason, you just switch to your previously determined alternate.
In the movie the weather is below VFR minimums, so I would expect the pilots to simply switch back to Center frequency & request an immediate diversion to their alternate.  A few might hang around for a short period, but I'm confident all would leave long before they were in any danger.
As noted in the question (Final reserve fuel) there's no major concern about running out of fuel.
On a side note, I've always disliked that movie - the premise that pilots are that stupid goes way beyond "suspension of disbelief".

the premise that pilots are that stupid goes way beyond "suspension of disbelief" -- Idunno, pilots do some pretty stupid things, particularly looking at the number of fuel exhaustion/fuel starvation incidents in General Aviation. (I do agree that in this particular film the pilots are displaying stupidity far in excess of what I would normally ascribe to them. Particularly for airline pilots, who should have diversion criteria beaten into their brains during training.)

@voretaq7 Some pilots are that stupid and worse (e.g., this, this and this). But the movie sort of implied that all the pilots were, across the board.

"I've always disliked that movie" -- I guess there's only one way to learn what it's like to be a cop watching all movies ever, and that's to watch a movie made in which your profession acts stupid. As a computer programmer I'm doing all right, in movies we're socially stunted brainiacs.


text:
What is the effect of wingspan on wing stiffness?
In an article in aviationweek about flight testing the GE9X, this statement is made.

To reduce aeroelastic interaction between the testbed and the big
  engine, GE has also stiffened the wing by removing the 6-ft. tip
  extension developed for the standard -400 series. The reduced span of
  just over 195 ft. will reduce potential exposure to limit cycle
  oscillation and maintain an unrestricted test envelope.

How does removing the wingtip extension make the wing stiffer?
What is "limit cycle oscillation"?

There is an aerodynamic force on the tip of the wing, which acts as a lever on the rest of the wing. Think of it this way, if you cut the wings in half, would you find it harder or easier to bend by grabbing the end and pulling?

Related: How are aircraft wings protected against flutter - aerodynamic oscillations that can break bridges?

Limit cycle oscillation is a fancy term for flutter with constant amplitude. The limit cycle is an attractor, meaning that the frequencies and amplitudes of a range of oscillations will converge to the limit cycle oscillation as time progresses. One example can be witnessed in this video.
By removing the wingtip, GE could raise the bending eigenfrequency of the wing, like you make a pendulum swing faster if you shorten it. Adding a heavier engine before certainly reduced the eigenfrequency, so the shortening was a way of compensating for the changed dynamic characteristics of the modified wing-engine combination.
Wingtip removal does not make the wing stiffer, only larger spar caps, a thicker spar or employing a stiffer spar material would. Stiffening the wing spar would raise its bending eigenfrequencies, and removal of the wing tip would do the same by different means. I guess the author wanted to express that bending frequencies were raised but picked the wrong explanation.

@jjack: Wouldn't it be better if we define terms in clear and concise ways and then stick to that definition? They did not change stiffness: With the same load over the same surface, the short wing will show the same displacement as the longer wing. You weren't the author of that poor article by any chance? This would explain your contorted reasoning.

@jjack; Why else would you so desperately defend an untenable position? And I never claimed the surface is the same. My only claim is that when the same surface is loaded with the same force, the wing will deflect in identical ways. Which means that its stiffness has not changed. Is that really too hard to understand?

Stiffness is defined as the deflection resulting from a force load. An aircraft in straight steady flight has an upwards aerodynamic lift, bending the wing upwards. The longer the wing, the more the wingtip deflects as a result of a certain load. This is the reason why wings with a high aspect ratio bend a lot more.
Image source
Removing a bit of wing means that the wing area is reduced and that the aircraft flies with a higher AoA at a certain speed and altitude. It also results in a shorter moment arm for the bending forces, hence less deflection of the wing tip at the same load, hence a higher stiffness.
Wing stiffness is not only defined by the construction of the wing, but also by external influences. This link to @DeltaLima's answer to a similar question provides more background.
Limit cycle oscillation is a form of sustained flutter. From this website:

We discuss the origin of different kinds of limit cycle oscillations. Limit cycle is a trajectory for which energy of the system would be constant over a cycle - i.e. on an average there is no loss or gain of energy.


text:
Are non-military aircraft allowed to fly with no registration numbers (tail numbers)?
There have been several news reports about aircraft (light planes and helicopters) flying without visible registration numbers or identifying marks in the USA:

The NYPD referred to the helicopter only as “23”—a reference to the
  number of police officers killed in the 9/11 terrorist attacks—and
  initially the aircraft had no registration number painted on its tail
  or side. source

and

Cimbolic was beginning to worry that he had overreacted when he
  noticed, on the same flight-radar Web site, the second plane flying
  higher in the sky, carving bigger loops above West Baltimore. The Web
  site reported that this plane was a Cessna 560 Citation V, a small
  jet. But it showed no tail number, offering no possible trail to
  Federal Aviation Administration records. This only heightened his
  curiosity. source

Eventually these aircraft were identified (and in the case of the former, a registration number found). But the general question I have is:
Are non-military aircraft allowed to fly without visible registration numbers (tail numbers) in the United States?
Fine print: I am aware that military planes usually have visible tail numbers, this question is about non-military craft. So "non-military" in this case includes federal agencies and state and local police.

In the second article, where it says "it showed no tail number", I think that "it" is referring not to the aircraft itself, but to the FlightRadar24 website.  In other words, I don't think they're claiming that there wasn't a registration number painted on the plane, just that FR24 didn't show it.  I believe that FR24 lets aircraft operators request that their registration numbers be "blocked" and not displayed on the site, if they prefer to have more privacy in their movements.

Thanks, Nate, for the clarification. Even if the news reports are wrong (that the planes did have tail numbers), I wanted to know if it were even possible that there could be planes with no tail numbers.

@Nate, it may not even be blocked. FR24 just may have, for whatever reason, failed to get the mapping from the identifier sent by ADS-B to the registration.

14 CFR 45 subpart C governs the display of "nationality and registration marks".
14 CFR 45.21 says:

Except as provided in §45.22, no person may operate a U.S.-registered aircraft unless that aircraft displays nationality and registration marks in accordance with the requirements of this section and §§45.23 through 45.33.

Section 45.22 gives some exceptions for "exhibition and antique" aircraft, and for aircraft of unusual design that may need an alternative marking scheme.
I presume there is another regulation, or international agreement, requiring that foreign-registered aircraft must also display their registration marks, but I haven't found it yet.
As to which aircraft have to be US-registered in the first place, see 14 CFR 47.  It's a bit complicated.
The US military is generally not bound by FAA regulations (though they may choose to follow some of them), see How much jurisdiction does the FAA have over military aircraft?.

Yes, I know the military isn't bound by FAA which is why I bracketed them out of this discussion. Thanks for the FAA regs. It'll be interesting to see how the agencies who are operating the unmarked craft are getting around this.

@RoboKaren: The first article claims "the NYPD also requested special “undercover” registration handling from the FAA..."  I would presume the FAA has the authority to waive any of its regulations on a case-by-case basis.

@NateEldredge Generally yes, provided that the regulation isn't actually directly written into a law that provides them no such authority.

@RoboKaren All military aircraft I've been around have side numbers and buno numbers displayed on the exterior of the aircraft.

The government in general is not bound by FAA regulations, not only the military.

I fly a gyroplane without the N number. It's an Ultralight that also has a N number, but since I fly it as an ultralight I need not display the N number...

Yup, Part 103 ultralights aren't required to have a registration number. I'm a little curious, though, since you say it "also has an N number, but I fly it as an ultralight". Does your gyroplane have an airworthiness certificate? Under Part 103, an ultralight must not have an airworthiness certificate; if an aircraft has an airworthiness certificate, it can't be flown under Part 103. So if it does have an airworthiness certificate, and you want to fly it under Part 103, you need to surrender the certificate.


text:
After decades of solid research, mostly led by Robert J Englar, why is Circulation Control, the Circulation Control Wing not in use at all?
Any research on the CCW by Robert J Englar at Georgia Tech from the 60's into the 2000's.
Many papers have confirmed Englar's work and in many cases, advanced it.
This research has proven extreme lift for slower, safer airliner take-off and landing as well as reduced weight, noise and drag.
https://smartech.gatech.edu/bitstream/handle/1853/14123/naqvi_messam_a_200612_phd.pdf This paper covers much for the practical use of a CCW but also references Englar's basic idea as well as posting several references to Englar's research.  These can also be covered to help answer my question.

Reminder: The answer box is for answers. The comments box is to seek clarification on the question.

I feel that this question is incomplete without some mention of safety. Do all these circulation control systems require mechanical intervention, in the form of sucking or blowing along the span of the wing? If so, would a failure of these systems likely lead to an immediate, and possibly asymmetric,  stall?

Circulation may be controlled via either air blown through spanwise slots or horizontal-axis rotors, alone or in combination.
Experiments on wing systems go back at least as far as 1902 and since then almost every variation imaginable has been tried.
The problems come in the engineering implementation. Where investigations have reached the stage of flight testing, and in many cases sooner than that, the conclusion is always the same. Either you have a system of large air ducts threading through the wing structure and weakening it, or you have a spinning rotor to support along the span of the wing. Either way, the wing must be strengthened and that means more weight. Both approaches require a supplementary power source, more weight again. The mechanical complexity compromises development time, manufacturing and maintenance costs, and reliability. The thing may work, but the downsides of the weight and complexity are greater than the benefits; the technology is not cost-effective.
So I am not sure what evidence base there might be for claims of actual weight reduction.
If you really want super-slow flight technology, get a helicopter with VTOL thrown in for free or, if you are rich and desperate for somewhat higher speeds, a V-22 Osprey.
Who knows, the balance may change as technology advances, stranger things have happened.

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text:
What would be the likely repercussions of flying into the Air Force One temporary flight restriction area?
I just saw this video on YouTube where a guy flies his plane into Air Force One's temporary flight restriction.
It's very clear that ATC are not amused and that he's caused a good deal of trouble. What would likely happen:

To his license? (lose it?)
Would he be fined?
Would he be arrested?

treated as a terrorist likely

Can this question be generalized as violating any TFR?

Is there any difference between the DC TFR with and without the POTUS in residence? Or, more correctly, is there any difference between the AF1 TFR and any other TFR?

There is a regulatory difference between TFRs for presidential/government VIP movement and other TFRs. The former are typically classified as "National Defense Airspace", while other TFRs (e.g. airshow TFRs) may not be. (The practical difference is violating a "National Defense Airspace" TFR can get you shot down. Violating the other kind will "only" get you yelled at and possibly fined.)

@voretaq7 is correct - remember, TFRs were originally used to keep your average pilot away from firefighting and other aerial rescue operations, not VIPs or sporting events!

@egid When the only tool you have is a hammer... :-)

The answer to each of the points in your question is "Maybe".
A pilot wandering into a presidential TFR (and many other "VIP Movement" TFRs) potentially faces some pretty severe consequences. Quoting the standard language from one such TFR NOTAM:

THE FEDERAL AVIATION ADMINISTRATION (FAA) CLASSIFIES THE AIRSPACE DEFINED IN THIS NOTAM AS 'NATIONAL DEFENSE AIRSPACE'. PILOTS WHO DO NOT ADHERE TO THE FOLLOWING PROCEDURES MAY BE INTERCEPTED, DETAINED AND INTERVIEWED BY LAW ENFORCEMENT / SECURITY PERSONNEL.
   ANY OF THE FOLLOWING ADDITIONAL ACTIONS MAY ALSO BE TAKEN AGAINST A PILOT WHO DOES NOT COMPLY WITH THE REQUIREMENTS OR ANY SPECIAL INSTRUCTIONS OR PROCEDURES ANNOUNCED IN THIS NOTAM:
A) THE FAA MAY TAKE ADMINISTRATIVE ACTION, INCLUDING IMPOSING CIVIL PENALTIES AND THE SUSPENSION OR REVOCATION OF AIRMEN CERTIFICATES; OR
B) THE UNITED STATES GOVERNMENT MAY PURSUE CRIMINAL CHARGES, INCLUDING CHARGES UNDER TITLE 49 OF THE UNITED STATES CODE, SECTION 46307; OR
C) THE UNITED STATES GOVERNMENT MAY USE DEADLY FORCE AGAINST THE AIRBORNE AIRCRAFT, IF IT IS DETERMINED THAT THE AIRCRAFT POSES AN IMMINENT SECURITY THREAT.

That's a lot of MAYs, so what does it mean in practice?

The pilot will be detained and interviewed by law enforcement
May is a nice word, but in practice if a pilot flies into one of these TFRs they will be held and questioned once they're on the ground to find out why it happened. It's kind of a big deal.
The pilot will face some kind of Administrative Action from the FAA
Suspension or revocation of their certificate is certainly possible (especially if it's not the first time).
A stern talking-to from someone at the FSDO and/or a reexamination ("709 ride") is almost certain.
The pilot may face Civil Penalties (a nice way of saying "fines")
$10,000 used to be the magic number for these fines as defined in 49USC, the law now refers back to 18USC (which I'm not going to read through because reading the US Code makes my head hurt), but I assume it hasn't gone down. The point is it could be an expensive error.
The pilot may face Criminal Charges (arrest and imprisonment)
I can't recall a case of a pilot accidentally violating a TFR and being locked up for it, but I imagine willfully violating a TFR could land you in prison pretty easily.
Use of deadly force may occur (the pilot may be shot down)
Again, I can't recall a case where a pilot has violated a TFR and been blown out of the sky by a missile from an F-16 (though there are plenty of cases where aircraft have been intercepted and diverted), but it is an option available if the aircraft poses an "imminent security threat", however that determination is made.

I recall a civil aircraft shot down by military when they flew over a ship (that was out of position)

This news article describes a real incident that happened in Seattle.
I don't know if he was punished much beyond some remedial training and a black mark on his record.

Anecdotally, I actually heard the sonic booms as the fighters screamed up to Seattle from Portland. Pretty weird event.


text:
Can a commercial airliner sink to the bottom of the ocean?
Is it possible that a commercial airliner could hit the ocean surface at such an angle that it made a deep dive intact, got filled with water, sank, and got stuck in mud at the floor of the ocean?

Intact. No. Planes dont do well on hitting the water. At the velocity with which it would fall coupled with a steep pitch, it wouldn't matter if it was water or ground.

Answers will be purely speculative, and without any value. "Yes" would be my answer.

@DeltaLima I don't think answers will be purely speculative.  GregHewgill gave a good answer with explanations.

@gerrit US Airways Flight 1549 landed more or less intact on the water and was "docked" before it sank. In the ocean it would eventually sink, possibly dive nose down and get stuck in the mud.

Historical note: this question was motivated by the disappearance of Malaysia Airlines flight 370.

No, it can't be intact. An analysis from CNN explains every possibilities. http://edition.cnn.com/2014/04/10/world/asia/malaysia-missing-plane-wreckage/index.html?hpt=ias_t2

Perhaps slower-moving A/C would have better possibilities of going down in one piece? Thinking of an ATR-42 or ATR-72 turboprop, with the straight wing and engines relatively high above the water surface.

Can a large modern jet airliner plunge intact underwater from flight?
No.
Large airliners are not desgned for this and are not even strong enough to make a landing on water in less than perfect conditions.

E.g. 961
striking the surface at a larger angle would certainly lead to widespread disintegration.
Is it possible for a large modern jet airliner to sink intact?
Yes (as near as makes no difference)
Almost all large aircraft impacting the sea surface in an emergency or uncontrolled will break up immediately and catastrophically.
One notable exception was US1549, an A320, which was landed on water without breaking up. It was described as "still virtually intact though partially submerged and slowly sinking". The left engine detached and sank.

So a controlled landing on water is possible under extremely favourable circumstances. It would then be possible for the aircraft to sink in more-or-less one piece without creating a large amount of floating debris.

Keep in mind that the belly of US1549 suffered fairly severe damage from the impact, which was the main reason it sank so quickly.

Also, Ethiopian 961 hit the water at a much higher speed than the vast majority of ditchings, and the pilots were fighting with a group of hijackers for control of the aircraft at the time, with somewhat detrimental effects on the aircraft's attitude and direction at impact.

Hitting water at speed is a lot like hitting concrete (try experimenting with different positions hitting the water off a high diving board—some will hurt). Aircraft are simply not designed to withstand the sudden forces introduced upon impact with water, no matter the angle. If an aircraft were to hit the water at high speed, the relatively flimsy structures of the aircraft would shatter, disintegrate, and completely lose structural integrity.

not to mention intact planes are naturally buoyant so they'll move up to the surface quickly

Note: don't try experimenting by jumping off a high dive unless you have really, really, fantastically good health insurance.

@ratchetfreak Note that my question involved the plane getting stuck at the ocean floor somehow.  But I get that many parts would break off on impact.

@gerrit Yeah, assuming the plane did magically stay together (well, magic or aliens) the ocean would have to be awful awful shallow for the plane to actually reach the mud and get stuck in it.

@JayCarr Would it still float if it became completely filled with water (I suppose it would)?

Nah, if it completely filled with water, it would sink.  But, remember, you are asking about a craft that has retained it's structural integrity.  If it was still in one pieces with a few minor cracks here and there, it would take some time for it to fill up with water.  Initially the plane would be quite buoyant.  So it would have to somehow overcome that buoyancy to get to the mud...  A possibly more likely scenario is for the plane to have ditched and, with the transponder dead, for it to have sunk before it was found.

@JayCarr: Note that a plane touching bottom before vanishing beneath the waves has in fact happened!

My answer is: It depends. As always.
Generally, yes, it is possible to land an aircraft on water intact. But the aircraft has to help you with that. If the engines are not below the wings (turboprop, piston, jet at the rear fuselage), and the pilot flares to minimum speed right above the water, gear up, then most aircraft will stay intact. Many are even tested for ditching qualities during development. I personally know of the ditching tests of the Breguet Atlantique and the Antonov-70, and there are certainly more (see https://www.youtube.com/watch?v=jK8ydLY5QHQ at 2:15 min into the movie).
A Dutch Atlantique once had to go down and landed intact in the North Sea. It was towed into a harbor and lifted out of the water a few days later. The openings (for cooling air etc) have special valves which close automatically when the aircraft is in the water, much like a snorkel on a submarine (sorry, could not find a web source for this).

It would be judged upon the state of the aircraft once stopped after the collision, taking into account velocity and angle of impact etc.
If the A/C is in-tact and no water is taken on-board it will float for a greater time but I highly doubt it will for a large amount of time because water always finds entry points.

If the plane flew to empty, and the pilot made a manual controlled glide and water landing, he would still strike the water at a speed in excess of 150 mph.
The engines would still be ripped off, baggage hold doors would most likely open on impact. There would have to be some kind of debris field. The plane would sink, but not intact.
Something would come free and float to the surface.

I doubt the baggage hold doors would open on impact if a normal ditching were performed. Engines breaking off is likely, but I think the cargo doors would likely remain closed. They have some pretty serious locking mechanisms to keep them from blowing out in flight. As far as I know, no cargo doors failed in the U.S. Air flight that ditched in the Hudson, for instance.

@reirab: Actually, the US1549 cargo doors did come open.

@Sean Interesting. I stand corrected.


text:
Are there any Airworthiness Directives for the Boeing 777 that might explain MH370's disappearance?
Are there currently any ADs for Boing 777s which would indicate the potential for catastrophic electronic system failures that might explain MH370's transponder failure and other issues?
Was the aircraft used for MH370 in compliance with all ADs?

This isn't really a question we can answer for you - While (FAA issued) ADs are a matter of public record information on AD compliance for each airframe is not publicly/centrally recorded: it's part of the aircraft's maintenance records, so you would need to talk to the maintenance department for Malaysia Airlines. (This information would also be contained in an NTSB investigation report, but at this point there is not even an NTSB file showing in their database.)

@voretaq7 that is assuming NTSB gets them

@ratchetfreak Indeed. Though I can't think of a case in recent memory where anyone refused to provide logs and records to the NTSB.

NTSB is not in charge of this investigation (though I understand they are or were assisting). Would they still have a public file for that?

They could have also fabricated maintenance records, which makes it more difficult to ascertain whether or not ADs were truly complied with.

@dvnrrs Generally any time the NTSB is involved in an investigation there will be an NTSB file number (with varying degrees of completeness in the information available - particularly in foreign investigations. e.g. this one says "Go ask the Australians"). Of course there generally has to be information to put into such a file...

Are there currently any ADs for Boing 777s which would indicate the potential for catastrophic electronic system failures that might explain MH370's transponder failure and other issues?

No one of the ADs issued for 777s can by itself fully explain all the facts of the final flight of MH370/MAS370.
Basic facts

The aircraft was a 777-2H6ER with 53460 flight-hours and 7525 take-offs and landings
A 777 has three VHF radios, two HF radios and two transponders.
Modern airliners have two main generators, an auxiliary power unit(APU) and a ram-air turbine(RAT) for emergency power. I think the 777 also has backup generators and two independent main power buses.
The aircraft left Kuala Lumpur for Beijing on 8 March 2014 at 00:14 local time with 239 passengers and crew on board.
Communications were normal up to and including 01:19 when someone on board (believed to be the co-pilot) said to ATC KL "Good night, Malaysian three seven zero". This was the point at which ATC control is handed to Vietnamese ATC. After there were no further communications from the transponder, the ACARS system or from the VHF radios.
Malaysian military reported that their primary radar system records do contain information on an unidentified aircraft to the northwest of Malaysia. The timing, position and heading of the unidentified aircraft in this report are consistent with MH370 changing course away from its intended flight path.
MH370 did not arrive at Beijing at 06:30 as scheduled.
A satellite communications system on the aircraft continued to respond  to routine hourly handshakes from an Inmarsat ground station via satellite 3-F1 located at 0.0°N (±1.59°), 64.5°E at a height of 35786 km. These normal handshakes ceased after 08:11, there was an unexplained incomplete handshake at 08:19.
Subsequent analysis of the handshakes is consistent with MH370 continuing into the southern Indian ocean (and somewhat inconsistent with other routes or it being stationary)
Countries within range of the plane's potential flight paths were asked to check their primary radar records for signs of MH370. No country reported finding anything.
No part of the aircraft or its contents have been located and firmly identified.
Many months later, some parts from the aircraft were washed ashore.
Investigators report that the condition, timing and location are consistent
with ocean currents from a point in the south Indian ocean.

Electrical failure hypothesis
Anyone speculating about the fate of MH370 needs to fit their hypothesis to the above facts.
They would have to explain how the failure incapacitated multiple systems within a few minutes and before any of them could be used.
In particular what did NOT happen:

The pilots did not make an emergency broadcast on VHF
The transponder did not squawk any emergency code
The ACARS system did not continue normal communication
The aircraft did not return to KL and follow procedures for loss of communication.
The aircraft did not continue on its planned route
The aircraft did not arrive at any of the airfields a pilot should divert to in emergency.

And what DID happen

The aircraft remained airborne for a further 7 hours.

Therefore the engines continued to run

Electricity was available to a satellite communications system on the aircraft.
The aircraft most likely deviated from its planned route.

AD 2014-05-03
This AD issued Feb 18th was reported on in March in connection with MH370.
But Reuters reports that Boeing stated this AD does not apply to the aircraft used for flight MH370.
If you ignore that and look at the consequences of a fuselage crack in the vicinity of the satellite antenna, you can see that this would not turn off the transponder, would not affect the VHF aerials and, if it caused a slow decompression - does not by itself explain why the crew would have failed to respond to the resulting low-pressure alerts by donning oxygen-masks, contacting ATC and descending to a safe altitude.

Occams razor
A shortage of facts always leads to speculation. Some of which can be very imaginative. The onus of proof is on those producing the hypothesis.
I'm sure there are thousands of hypotheses but, in my opinion, the simplest explanation I've read which is consistent with the known facts is deliberate action by a pilot or persons unknown. This doesn't mean it is the right explanation, just that it seems the most probable to me at the moment.

Was the aircraft used for MH370 in compliance with all ADs?

We won't know with any certainty until the investigations branch of the relevant regulatory authority publishes their report on the incident. In previous accidents over an ocean, it has, on at least one occasion, taken years before a final report could be written.
Currently, there have been no confirmed reports, that I can find, of an AD that warned of the potential for catastrophic failures and which should have been applied to 9M-MRO and wasn't.

@RedGrittyBrick, you do not have the ability to determine whether AD's for 9M-MRO were in compliance or not given that avionics records for MH370 were destroyed by arson on 26 March 2014... http://weechookeong.com/2014/03/26/azhari-explain-mysterious-fire-in-avionic-shop-of-mas/

@user2357: and nowhere did I say that I did.

@user2357: please provide any evidence/reference that "records for MH370 were destroyed by arson". That's an unsubstantiated rumour only. MH370 is not even mentioned in your link.


text:
What would be a safe minimum vertical separation distance between passenger-carrying multicopter aircraft?
There is increasing talk about employing passenger-carrying multicopters ("taxi drones") to extend urban mobility into the third dimension. These aircraft will have MTOMs in the range of several hundred kgs.
In case this catches on on a larger scale, airspace use will increase considerably compared to the current occasional helicopter and sight-seeing flight.
For safety reasons, presumably a certain minimum vertical separation will be required due to the downwash from the rotors.
Is there any information available about the current thinking on required minimum vertical separation between manned/passenger-carrying multicopters? Also, will there be any difference for this requirement between hovering and cruise flight modes?

The downwash is directly related to total lift; more rotors just means each rotor is producing a smaller fraction of the total, and the end result should be no different for separation purposes than for one or two rotors.
There are a lot of obstacles to mass adoption of personal aircraft, but the exact lift mechanism is arguably the smallest.

Thanks for the comment. I realise the question was maybe not clear enough, and I agree that the exact number of rotors in not relevant. So, the question was: What is the required minimum vertical separation between two multicopters, especially with regard to the downwash from the rotors.

ICAO rules require a minimum of 500ft vertical separation between all civil aircraft; helicopter downwash is irrelevant at that distance. This is more due to altimetry margin of error than aerodynamics. Wake turbulence can be a problem at several times that distance, but it shouldn't be an issue between two aircraft of similar size.


text:
What restrictions are there to building a tower near a private airstrip?
I'm exploring the possibility of putting up a radio tower on a property I own. The problem is that my property is roughly 1,000ft from a private airstrip. The location that I want to put the tower is not within the approach/departure path of the runway and is not under any navigational easements. The height that I'm looking at is roughly 150ft, just high enough to get approximately 20-30ft above the existing trees in the area. My question though is, what are FAA restrictions around building towers near private airports? Is it different from public airports? If so, is it possible to do this at all so close to an existing airport?

The FCC has a web page that you can fill out that tells whether you have to register the tower with them.

According to this page there are apparently some new regulations for towers 50-200 ft that goi into affect this month (Sep 2017).

According to this AC you dont need to light the structure,

Any temporary or permanent structure, including all appurtenances,
  that exceeds an overall height of 200 feet (61 m) above ground level
  (AGL) or exceeds any obstruction standard contained in 14 CFR Part 77
  should be marked and/or lighted.

But the FAA does have special cases and since you are close to a strip they may require/recommend it,

The FAA may also recommend marking and/or lighting a structure that
  does not exceed 200 (61 m) feet AGL or 14 CFR Part 77 standards
  because of its particular location.

Looks like you need some specific stuff to actually light it. Your local FSDO should be able to help out with what ever info you need. As far as I know these kinds of regulations don't change between public and private airports.
You can find the letter of the law for 14 CFR Part 77 here. The most applicable to your case is quite possibly,

(3) A height within a terminal obstacle clearance area, including an
  initial approach segment, a departure area, and a circling approach
  area, which would result in the vertical distance between any point on
  the object and an established minimum instrument flight altitude
  within that area or segment to be less than the required obstacle
  clearance.

Even though you are not directly in line with the runway you may be in conflict with a potential traffic pattern or approach segment.

It is also a good idea to notify the FSDO just because they can add a NOTAM for the obstruction in the AFD (or d-CS as they are calling it now).

Thank you for the detailed answer! Seems like it's not entirely outside the realm of possibility then. I'll see what the FSDO says about it.

I guess rergulations are different in Canada.

@KeithMcClary Ha! Yeah, I suppose so. In the U.S., that building permit would be denied... and if you built it anyway, I'd expect some bulldozers to be showing up in short order, especially at a public-use field.


text:
Did AirFone ever make money?
I remember when the first AirFones were put into service in 1990s and marveled at who would be rich enough to pay $7/min for air to ground phone service.
Even though every seat eventually had an AirFone, I cannot once recall seeing one in use (except in movies) in twenty years of flying (in economy).
I know they're being pulled out now, but did AirFones ever make a profit? Who paid for their installation, was it the airlines or did GTE etc pay for everything?

I imagine onboard Wi-Fi will be used much more.  Especially by businessmen and women.

Not according to the authors of The 4 A's of Marketing 1

Despite impressive technological achievements and great expectations all around, the service’s business performance ultimately proved an abject failure. In the ten years before Verizon finally pulled the plug, Airfone generated only 50 million total calls, a fraction of the calls carried daily by cellular companies. While the Airfone service was heavily used when bad weather caused significant delays, the system’s utilization at other times was extremely low. A typical plane was equipped with as many as 60 phones. From these, the average large jet generated fewer than 100 calls per day in about 16 hours of flying time. As a result, the expensive system, with its heavy load of fixed costs, remained idle well over 99% of the time.
...
While Airfone claimed that some customers’ monthly bills topped out
  at $4,000, there were not nearly enough of them to sustain the service’s sky-high fixed
  costs.
...
Within a few years of the service’s launch, airlines started to remove the expensive phone
  infrastructure from their cabins, since the Airfone cost them more in increased fuel
  consumption (because of the system’s weight) than it generated revenues through their
  revenue sharing arrangement with Verizon.

My emphasis.

Perfect answer. Interesting that at 100 calls per 16 hours, that's still 6 calls an hour per plane. Maybe it was all front of the cabin folks.

I can't imagine many people are sitting in economy but paying $70 for a ten minute call?

One wonders why they bothered to put the phones in the back of the plane anyway.

I wonder why they never simply dropped the rates to be affordable. Even if they weren't able to get to a point to make money, they would have been losing less.


text:
What are the pros and cons of a lift fan versus a helicopter?
F-35 uses lift fan which can create 20,000 lbs (9 tonnes) of lift in a small compact platform. I can see three or four of such lift fans that could lift a heavier plane and is seems better than a helicopter because it can travel at higher speeds.

So what are the pros and cons of each design?
Which one is more efficient in lifting, lift fan or rotate wing?

To answer your second question first:
Generally, it is always better to accelerate a big amount of air a little than a small amount a lot. Therefore, the helicopter rotor is much more efficient when creating lift, similar to a propeller which is more efficient than a jet. In the same way, more wing span reduces induced drag since more air can be used for lift creation.
The advantages of a lift fan are mainly due to its compact size:

Low mass, low inertias
Compact size, therefore easy to tuck away for fast flight
Low torque for a given amount of lift

and the disavantages stem from the same source:

Low efficiency
High exit speed, so beware of the blast.
No autorotation possible when the engine fails.
Lift acts at the center of the fan, so you need additional jets for pitch and roll control. A helicopter can shift the center of lift away from the rotor axis (within limits) by using cyclic changes to the angle of attack of the single rotor blades.

You can't autorotate with a lifting fan

You typically need several lift fans and they need to be redundant if one fails.

@rbp: Good point! Thank you, I added it.

How about using a ducted fan for pitch and roll?

@rbp: The usual way is to have two main jets, one from the engine exhaust and one from a lift fan or dedicated lift engines. Pitch and roll control is done by simple downward-pointing tubes with rotary valves at the extremes of the airframe which are fed by bleed air. Replacing the lift jet by a moveable ducted fan would be mechanically very complex.


text:
How does the efficiency of a fixed wing compare to a rotary wing?
Is fixed wing more efficient than rotary wing?
If yes, why is that?

FYI "rotary wing" is helicopter and "fixed wing" is traditional plane

@ratchetfreak don't forget gyrocopters :P

Define efficiency.  A helicopter is much more efficient than a fixed wing when lifting logs from a remote clearing and putting them down on the other side of the hill.

See Also: Why are rotary wing aircraft less efficient than fixed-wing aircraft?

Helicopters use more fuel than airplanes (considering everything else being equal).
When helicopters are traveling slow, they need more energy to hold their weight. When traveling fast (according to their standards), the rotor drag is very high. This is the primary reason they need more power. More Power = More Fuel.
Conventional aircraft have much less drag at a given weight and speed than a helicopter.
Helicopters travel slow. When they burn more fuel per hour and travel slowly, fuel consumption per mile is much worse. They also stay low. Most have turbine engines, turbines burn lots of fuel at low altitudes.
Wikipedia has a good article about energy efficiency in different vehicles.

Two-seater and four-seater flying at 250 km/h with old generation engines can burn 25 to 40 liters per flight hour, 3 to 5 liters per
  100 passenger km.
The Sikorsky S-76C++ twin turbine helicopter gets about 1.65 mpg-US (143 L/100 km; 1.98 mpg-imp) at 140 knots (260 km/h; 160 mph) and
  carries 12 for about 19.8 passenger-miles per gallon (11.9 litres per
  100 passenger km).

Not strictly true.  Up until a certain point, power required drops significantly as airspeed increases.  Once translational lift is gained, induced drag drops with speed.

Fixed wing aircraft are generally much more efficient than rotary aircraft. This is because of the difference in how they generate lift.
For fixed wing aircraft, they use an engine to keep the plane moving forward. The air flow over the wings generates lift. The engine must only overcome the drag of the airplane (in level flight). This drag is both due to the form drag of the plane and the generation of lift from the wings.
For a rotary aircraft, the engine keeps the rotors moving. The rotation of the rotors generates lift, but this is also the main force used to move the aircraft forward. So in this case, the rotors not only provide the lift to keep the aircraft in the air, but also the thrust to keep it moving forward.
In a fixed wing aircraft, the propeller can be made to be efficient at providing forward thrust, and the wing can be made efficient at providing lift. In a rotary aircraft, the rotors must serve in both roles, and result in a compromise.
When a helicopter enters a hover, it is using all of its power to provide lift, while a fixed wing aircraft can enter a holding pattern with still only enough power to offset the drag.
Comparing the efficiency of lift versus drag of different aircraft shows the lower efficiency of a helicopter. A Cessna 150 has an L/D (lift to drag ratio) of 7 in cruise, while a helicopter would only have around 4.5 in cruise. Other sources show that small aircraft can achieve an L/D of over 10.
There is also a speed limit for helicopters that prevents them from going as fast as most turboprop or jet aircraft. This is because the tips of the rotors should stay at subsonic speeds. Current helicopters like the UH-60 are limited to around 200 knots. Newer designs such as the Eurocopter X3 and Sikorsky X2 are capable of up to 250 knots, but are still in development.
For comparison, a small helicopter and small fixed wing plane. Even though the PA-31 (fixed wing) is heavier than the 206 (helicopter) and can carry more passengers, it can fly faster and further than the helicopter. The "fuel economy" of nautical miles per gallon is based on the fuel capacity and max range, so it's not an exact figure of actual fuel burn.
Also, the PA-31 ended production in 1984, while the 206B-L4 is the latest version still in production.
Bell 206B-L4

1 pilot, 4 passengers
Max speed 120 kts
Range 374 nm
Fuel 110.7 gal

3.4 nm/gal

PA-31 Navajo

2 pilots, 7 passengers
Max speed 227 kts
Range 1011 nm
Fuel 187 gal

5.4 nm/gal

It's somewhat misleading to claim that, in a fixed wing aircraft, the engines only have to overcome drag and move the plane forward, while the wings provide the lift. Wings generate lift by doing work against the air; the equal and opposite reaction to doing that work is (part of) drag; the power to do all of that work comes from the engines.

@Articuno gravity is only doing work on the plane if it is descending. Gravity doesn't do work on a plane flying at constant altitude, just as it doesn't do work on an object that's sitting on the floor: work equals force times distance moved in the direction of the force, which is zero in both cases.

@DavidRicherby : You appear to be referring to induced drag. Induced drag is included in L/D figures. As fooot both gives L/D figures and writes "this drag is both due to the form drag of the plane and the generation of lift from the wings", it is not clear to me what sort of misunderstanding you are positing.

@sdenham The L/D figures and the statement you quoted were added in response to that first comment.

@DavidRicherby , fooot : I didn't consider that possibility, and in that light, I see that David's comment was relevant - the answer does depend on the actual values of L/D that can be achieved. High-performance sailplanes have best L/Ds in the mid 40s-to-1 and dissipate less than 2hp at the corresponding speed, which I think is pretty amazing. Adding a motor increases the drag (including a contribution from the increased induced drag from having to lift an increased weight), but some motorized gliders are claimed to have best L/Ds above 30:1.

Opinion;
The lift to drag ratio of fixed wing versus rotary wing by logic costs the rotary wing
greater fuel because energy is expended in lift, where that is not required in a fixed wing.
The most efficient practical fixed wing situation is " wing in ground effect" where less
energy is required to generate lift due to air compression beneath the wing. Here, the better efficiency is obtained with a larger wing by adjusting speed to maintain lift.

Can you please add sources for your claims?


text:
What is the drag of helicopter rotor at jet speed?
I wonder if a jet plane can have helicopter rotor for vertical take off and then, when moving forward just stops that rotor and uses lift from wings like normal plane.
Since rotor blade seems thinner and smaller than wings, I guess when turned off, its drag should be much lower than wing.
So why doesn't that type of hybrid plane exist?
Or the drag of the rotor at the jet speed would be so high? If the drag is high, why is it high, since the rotor blade is also airfoil like wing?

Which weight for the aircraft? Because usually helicopters are very limited in weight, due to the rotor being a small wing with limited lift, and the mast bearing all the forces.

For your general problem, please take a look at the Sikorsky S-72 X wing experimental program.

There were actually some experiments with stopped rotor. Furthest with the actual implementation was probably the Boeing X50 Dragonfly:

By Source, Fair use of copyrighted material in the context of Boeing X-50 Dragonfly

The main reasons I can think of why nothing like this reached production yet are:

When the rotor spins, the blade on one side moves aft and needs to have its leading edge face aft, but when the rotor is stopped, both sides need the leading edge facing forward. So either the airfoil must be symmetric, but that has really poor efficiency, or some blades have to turn somehow, which is complicated.
To be light, rotor blades are normally very flexible and rely on the centrifugal force to keep them straight under load. But when they stop, they won't have such force, so they must be much stiffer, which makes them bit too heavy to spin.
Having separate wings and rotor would add a lot of weight, especially since the rotor would still have to be made strong enough to resist the aerodynamic loads when stopped.

Reading the Wikipedia article I would say the main reason this design failed is the effect of the rotor downwash on the fixed wings. This aircraft in fact created its own massive amounts of turbulence and hence never made it to level flight in one piece.

@RobVermeulen, the reasons I write are the ones that make this approach difficult. This particular experiment did indeed fail mainly due to aerodynamic instability.

So why doesn't that type of hybrid plane exist?

This type of plane does not exist largely because there is a simpler way to go about this that has already been done with some success. Planes like the Harrier are capable of VTOL by simply re-directing the exhaust of the jet.

(source)
In this case you reduce the amount of moving parts by not needing both a regular jet system and a helicopter mechanism (which requires a lot of up keep) instead its simply easier to have a directed nozzle

(source)
This idea has been carried over into the prop world with Boeings V-22 Osprey which has tilt control rotors that allow for VTOL but are also used for forward flight. In this case

(source)
To directly answer the drag question, yes the drag would be high (some one else may be able to provide a specific number or the math) but like anything else the design could be built it just would not be very efficient. Since you can generally control the pitch of a helicopter rotor you could potentially (on an even numbered rotor system) orient at least 2 of them to act like small wings and generate very limited lift to assist you even in their static situation.
Like most things in aviation planes are built for a given mission profile. VTOL fits certain cases like anding on an aircraft carrier or in other hard to reach places but things like commercial planes already have a large runway based infrastructure to support them. Any hard to reach place that you need to land at most likely also means your loads are small and a helicopter will suffice, you would need a large hard to reach place that had constantly large loads going in and out to justify a sizable commercial plane that had VTOL capability. While there are some applications for this in the military space (carrier landings come to mind) there are other systems that have been found to be efficient to use. Interestingly enough the directed exhaust from the new F-35's has caused issues on the deck of aircraft carriers.

Suggest you add S-97 and Sikorsky X2 pictures, if  you are going to add Osprey.

Anything that uses a rotor to take-off vertically is most inefficent when lifting, hovering or moving slowly.  This is most obvious when a helicopter is at max weight or in "hot and heavy" conditions when it might not be possible to take-off vertically.
In order to lift vertically, thrust must be greater than weight.  This is not true for a "conventional" aircraft.
If you use a rotor only for lifting off, then that rotor becomes nothing but extra weight (and complexity) that you don't need.  Maybe you could use the stopped rotor to generate some lift in a similar fashion to a conventional wing but as others have pointed out, to make a rotor blade which is rigid enough to act as a static wing and can still handle the forces generated when rotating is a serious engineering challenge that will add more weight and complexity (because the rotor hub must have hinges that can absorb the forces when rotating but keep the blade rigid when stationary).
You also need some complex, and therefore heavy, mechanism for tranisitioning the thrust from the rotor whilst lifting to whatever propulsion you use for forward flight.
More weight, more complexity, more maintenance, more cost, more to go wrong.  What's not to like? ;)
The other answers also point out better solutions such as vectored thrust and using the same rotors to lift and to provide thrust in forward flight.
Add this all together and you have a solution for a problem which doesn't exist.

See "ABC" and "X wing" development in the 70's and 80's.  See also recent Sikorsky X2 and S-97 with a pusher prop to overcome some of the issues, and of course raise other challenges (to include scaling, as they hope to do with Defiant).


text:
Are CO2 cartridges not considered explosive?
I've recently starting riding a push bike to work and got myself a CO2 pump and some cartridges (like this), which I promptly forgot about. As a result, when going on work trips and holidays, I had a cartridge in my bag when going through airline security.
I estimate roughly 15 airports (some duplicates) either didn't detect the cartridge or just ignored it. Perth International is my main airport, but I also visited Singapore 3 times (so bare minimum of 6 security checks there. Is this normal behavior/protocol? I would have thought that while CO2 cartridges are single use, they probably present more danger than other prohibited items, such as nail clippers.

I use those too, and when I went through security with my front (Carbon Fiber Aero) bike wheel they didn't question the cartridges.

I had a cartridge in my bag when going through airline security.

These are not uncommon. They are also used in for instance:

Life Vests
Ski Airbags
Soft air guns

In other words, it's not an unusual object, especially for holidays which I guess both your two mentioned destinations might be for many travellers. I've checked in my bag with an automatic self-inflating life vests a few times and while it went in the  hold never had any problems or found it removed. I believe there may be some limitations though on the number you can pass through, and they might start asking questions if you have more than two.

I estimate roughly 15 airports (some duplicates) either didn't detect the cartridge or just ignored it.

It's not unjustified not checking them in. Onboard pressure changes, the cargo hold is cold and will shake during turbulence which just might trigger it to deploy if left screwed onto a life vest of similar.

probably present more danger than other prohibited items, such as nail clippers

I can't see any real threat posed by these, except for the gun application, which of course requires a gun and not only a cartridge. Btw, TSA allows nail clippers.

Ryanair, the World’s favourite airline ;) states the following:

Carriage of Self Inflating Lifejacket
A self inflating lifejacket with 2 carbon dioxide cylinders (plus 2
  spares) can be included in either cabin or checked baggage.

In other words, they appear pretty liberal on the matter.

Ah, so not as uncommon as we all thought. Did not think of life vests, etc. Thanks for the detailed answer, much appreciated.


text:
Are passengers really not allowed to change seats before take-off due to weight and balance?
Just boarded an A320 with many open seats (Spirit), and the flight attendant said we may not change seats until at altitude due to weight and balance.  Is he just trying to prevent pax from moving around while the crew is busy moving around the cabin, or is this a legitimate issue?  I feel it must be the former, in which case I feel they're being a bit deceptive.

There's no way to know if your particular flight was a true weight and balance issue. It can occur with various imbalances of passengers, fuel, and cargo, but without looking at the raw numbers its hard to come down one way or the other

Just to confirm the question I had the same issue a couple of days ago on a KLM A330-300. It seems that this applies also to large aircraft.

Related: How does a commercial airliner measure its weight/mass?

In some situations, on some aircraft, passengers moving from their assigned seats might cause a CofG problem in some flight phases.  Now, we have 2 options.  Train the cabin crew how to be able to calculate the risk of a problem for this specific flight, from this specific airport, on this specific departure, with this specific load or, train them not to allow passengers to reseat themselves until in the cruise, then ask them to return to their assigned seats for arrival.  Which one would you go for?

Example of A320 that had problems due to passenger positioning

Related: Is passenger weight distribution important on a commercial flight?

The crew gave you the correct reason, but it would had taken a co-ordinated effort from the majority of the passengers to upset the aircraft's balance.
But in smaller aircraft even a single person moving around can shift the center of gravity outside its safe limits. The de Havilland company once produced such an aircraft, the DH86 Express. Wikipedia reports:

On 2 October 1935 Holyman's VH-URT Loina was also lost in Bass Strait,
  again with no survivors. This time a significant amount of wreckage
  was recovered from the sea and from beaches on Flinders Island.
  Investigation of the wreckage revealed a section of charred carpet on
  a piece of cabin flooring from just ahead of the lavatory door. It was
  thought possible that a small fire from a dropped cigarette had led to
  someone running aft suddenly to stamp it out – the sort of sudden
  change in weight distribution that could set up a fatal loss of
  directional control while the aircraft was on a low-speed landing
  approach.

A dramatic example what a shifting load can do to the controllability of an aircraft was National Airlines Flight 102, a Boeing 747-400 freighter conversion which took off from Bagram on April 29, 2013. The most likely cause was insufficiently secured payload which shifted back when the aircraft pitched up during take-off. A single passenger will not be as heavy as a MRAP, but several of them moving over a significant part of the cabin length in combination with a position of the center of gravity close to the limit could well upset the flight.

CG at the forward limit, passengers moving to the front: The aircraft will need higher speed and much more elevator deflection for lift-off. Especially in combination with an engine failure late in the take-off run, a crash is a distinct possibility.
CG at the rear limit, passengers moving to the back: The static stability and, in manually controlled aircraft, the stick forces are reduced. Flying the aircraft needs more attention, and in extreme cases the aircraft will become longitudinally unstable.

In any case, the pilots need to adjust trim.

The need to adjust trim is why this is more important before take-off. In flight, trim is set based on response of the plane. But before take-off, it is set based on calculated CG and if the calculation is off, the plane will behave unexpectedly on rotation. CG shifting aft is more critical as it can lead to over-rotating and tail-strike.

I remember travelling on a small commuter airline that flew Fairchild Metro IIIs.  You had to declare your weight at the check-in counter and they weighed carry-on bags before you got your seat assignment.

Another dramatic example, this time due to passengers: https://en.wikipedia.org/wiki/2010_Bandundu_Filair_Let_L-410_crash . The passengers rushed to the front because a crocodile escaped its duffel bag.

@Adam The crocodile theory should be taken with a grain of salt, as it seemed to appear only months after the crash and contradicts the original story told by that witness.

An airline crew has two primary goals: safety and legality.
Even for large airplanes, one person's change of position can put the takeoff CG outside legal limits and/or safe limits.  Either situation is unacceptable.  In order to calculate the CG, the passengers have to stay in a particular spot so the cabin crew can report the passengers' locations to the flight crew, who then calculates the takeoff CG.  The passengers should stay in the same seat from the beginning of this process until after takeoff and initial climb, and preferably throughout the flight.
I fly a CRJ-900, for example.  This is a 70+ passenger, 85,000lb regional jet.  Its not uncommon for us to ask the cabin crew to move one passenger from front to back in order for the calculated balance point (CG) to be in the correct envelope for takeoff.  If that one person moved back after the calculation, the CG would be just outside the CG envelope, and we would technically be illegal to take off, which is unacceptable.  Each flight prepares a manifest of passengers and calculated weight and CG which is kept on file and audited by aviation authorities like the FAA, and the recorded values must show a legal loading.
The CG envelope includes some safety margin, so being a tiny amount (0.1% MAC for example) out would likely not make the aircraft uncontrollable; but it is possible, especially if several people are moving, and its an unknown situation that no professional flight crew is going to permit.  Even on a "large" plane, one person's location has such an effect on CG that if someone in the front walks to the rear in flight, it will have an effect on the trim of the plane and either the pilot or the autopilot will correct for this using trim.
So yes, its likely the crew was serious and honest when they asked you to stay in one location for balance purposes.  It could have been for safety or legal reasons, and both are required to be maintained by the crew at all times.

During takeoff, cabin crew sit in designated positions, and their position and weight is calculated into the takeoff CG.

Are you telling me passenger placement is calculated for CG on aircraft like the 747 or the A380; and the crew are reporting to the cabin where the passengers are seated? Is this something that is only a factor during takeoff? I can't imagine the headache of CG on the A380 which has a bar and and a shower; and how long it takes crew to report each passenger's position prior to takeoff (assuming this is when CG is calculated for trim purposes).

@BurhanKhalid Yes, passenger placement matters for takeoff CG calculations for even very large aircraft.  The cabin crew isn't telling the captain "John Doe is in row 1A, his wife in 1B...."for 300 people.  The specific procedure differs between airline, some use a zone system, others a load reported by the gate agent, a load master, or some other system; which is beyond the scope of this question.  No matter how big the aircraft, the people in it make up a significant portion of the weight, and thus effect the CG, and can't be ignored in ensuring the takeoff can be made safely.

Obviously, one person moving from row 27 to row 28 on a 747 just at takeoff isn't going to crash the plane.  However, if 20 people move after the CG calculations have been made, there is no way to determine what the effect is, and in any case incorrect data will have been provided on the legal manifest.

I knew of the load manifest - tells the crew about the placement and type of cargo and their weights; and they have a flight manifest that tells them the number of passengers on board. I didn't realize they also have to know the placement of the passengers even on large aircraft.

Not each passenger, no.  For example, some airlines divide the plane into multiple zones, each zone represents a certain range of rows.  The passenger load could be reported as 35 people in zone A, 22 in zone B, 55 in zone C, etc.  Passenger weights are estimated, locations are estimated, etc; with enough of a "fudge factor" that the estimates either are insignificant or cancel each other out.  Example, the A380 can weight up to 1,272,000lbs, and with 644 passengers, the pax make up 10% of the takeoff weight (or more at lesser weight), plus cargo and fuel, so pax load is not insignificant.

From NLR Analysis of aircraft weight and balance related safety occurrences

Example 1: Overweight take off B727-200 (PP-LBY), Fly Lineas Aereas,
Quito airport, Ecuador, 01/05/1996. (source: NLR Air Safety Database)
During take off from runway 35 at Quito, the crew felt that the
aircraft was not accelerating quickly enough and was not reaching the
calculated V speeds. Therefore the crew elected to abort the take off
at 120 knots (V1=143 knots). The runway was wet and the available
runway length left to stop the aircraft was only 900 meters (3000
ft.). The aircraft could not be stopped on the runway and overran the
end. It came to rest some 130 meters from the runway end after having
struck an ILS antenna and the airport perimeter fence. The maximum
take off weight was exceeded by some 9,729 kg (+16%) for the
conditions at Quito. It was determined after the accident that the
crew had not calculated the weight and balance for the flight. Instead
they had used the load sheet from a previous flight.

Example 2: Exceedance of aft centre of gravity limit during landing
F27-600 (G-CHNL), Channel Express (Air Services) Ltd, Guernsey,
Channel Islands, United Kingdom, 12/01/1999. (Source: AAIB UK)
The
aircraft was destroyed when it went out of control and crashed during
the final stage of the approach to Runway 27 at Guernsey. After an
uneventful flight, during the final stage of the approach, the pilot
called for 'flaps forty' (the full down position) and the flaps were
extended to this position. Moments after the wing flaps were lowered
to their fully down position, the nose of the aircraft rose and the
crew were unable to prevent it rising further. The nose continued to
rise until the aircraft's pitch attitude was near vertical. Although
the crew applied nose down pitch trim and high
engine power, the aircraft lost flying speed, stalled and entered an
incipient spin. Returning the flaps to the intermediate approach
setting of 26.5° and raising the landing gear did not restore
controllability. It descended in a shallow nose down pitch attitude
with little forward speed and crashed at the rear of a private house,
striking the house with its port wing. Both the house and the aircraft
caught fire. The two pilots were killed but the sole occupant of the
house escaped without physical injury. The aircraft was operating a
flight from Luton with a cargo of three tonnes of newspapers. Prior to
departure, the cargo had to be hand loaded. However, neither the load
team leader nor the dispatcher had loaded an F.27 before nor did they
have a load plan to assist them. They therefore asked the captain how
to proceed. The captain reportedly replied along the lines 'from the
back' or 'put it all in the rear.' Subsequently the loading team
stacked the papers in even piles, some 2ft. 6in. high, across the
width of the cabin, working from a point in line with the rear doors
forward. The papers eventually extended forward for an estimated
distance of between one quarter and one third of the length of the
cabin. As a result, the aircraft's centre of gravity ended up
significantly aft of its approved limit and it became uncontrollable
once full flap had been selected for landing. The crew of the aircraft
appeared to have taken only limited interest in the loading. The
comments made by the investigators was that 'this behaviour contrasts
strongly with the commander's careful manner and thorough attitude
whilst actually flying' and suggests that 'either he was not aware of
the importance of load positioning and restraint or that he was not
sure how to direct and supervise the loading operation.' No official
'load planning' tables were provided for the flight crew to use. Crews
were apparently expected to devise a load plan by 'trial and error'
using the balance chart on the load sheet. The investigators commented
that this could be time consuming and not as error resistant as
pre-planned tables. Additionally, it was noted that loading procedures
were not a structured element of the command training syllabus and
there was therefore an element of chance that new commanders might not
be properly trained in this area. This accident was provoked by
operating the aircraft outside the cleared load and balance
limitations. This error went undetected because nobody ensured that
the cargo distribution in the aircraft was the same as that shown on
the load and balance sheet.

Example 3: Exceedance of forward centre of gravity limit during take
off Convair 880, N5865, Air Trine, Miami International Airport, USA,
16/12/1976 (source: NLR Air Safety Database/NTSB)
The Convair 880 was
loaded with a cargo of cows. Following an apparently normal take off
run on Runway 09L reaching the rotation speed, the aircraft would not
rotate despite repeated efforts by the crew including re-trimming the
aircraft to the 'full nose-up' position. The pilot
subsequently elected to abort the take off but was unable to bring the
aircraft to a stop before the end of the runway. After leaving the
runway, the aircraft passed over an area of soft ground, where its
nose undercarriage collapsed, before falling into a wide drainage
canal. The investigation determined that on take off the aircraft's
centre of gravity was some 2.2% of the mean aerodynamic chord in front
of the maximum forward limit, due to the way the aircraft had been
loaded, and that the crew's weight and balance calculations bore no
resemblance to the way the weight was actually distributed.

Example 4: Overweight take off with an exceedance of forward centre of
gravity limit B727-200, 3X-GDO, Union des Transports Africains,
Cotonou, Bénin, 25-12-2003 (source: BEA Bureau d'Enquêtes et
d'Analyses pour la Sécurité de l'Aviation Civile, report translation
3x-o031225a)
On December 25th, 2003, a Boeing 727 operated by the
Union des Transports Africains (UTA) crashed during take off from
Cotonou. There were at least 160 people on board and only 22 survived.
Passenger boarding and baggage loading was carried out in great
confusion. For flight preparation, incomplete information on the
loading was provided to the Captain. He had determined the
configuration for take off on the basis of this information. The
investigation showed that, after the brakes were released, the
aircraft accelerated up to rotation speed. As the forward hold had
been filled, the aircraft had a significant forward centre of gravity
that the crew had not compensated for with the stabiliser because they
had not been informed of the loading of this hold. The pilot's nose-up
input thus did not have an immediate effect and it took seven seconds
for the aircraft to leave the ground, with a very low slope angle. The
aircraft hit a building located on the extended runway centreline,
crashed onto the beach and ended up in the ocean. The investigation
also showed that, without the uncompensated forward centre of gravity,
the aircraft would have taken off despite its excess weight. The
investigation concluded that the accident was due to the crew's
difficulties in performing the rotation with an overloaded aircraft
with a forward centre of gravity that they were unaware of.

Welcome! These are interesting comments that should be either turned into a full answer, or posted as comments. You'll be able to post comments after you get 50 points of reputation (will be quick). For improving to a full answer, please add your analysis and include references to the original pages. More about good answers.

Wow this is some good research.  It's hard to read and draw a quick conslusion to answer the original question though, and this would be a high-quality answer if you did at least one of the following:  include a summary of what all these examples show regarding the original question, bold the most important part of each example, break long paragraphs at appropriate points, cut out unnecessary information and replace it with a link to find out more, and/or summarize each individual incident.  Also, including a link to your source would be helpful.

If it was before the takeoff they probably just wanted nobody in their way. If the weight and balance was such a serious issue, Southwest airlines would not have the ability to pick your seats anywhere on the aircraft.

It that your opinion, or something that you can support by a reference?

Southwest also knows they're going to fill their airplane.

It was probably BS and the attendant was just being overly cautious. They probably say that on any flight with a half-full cabin. There is no way to know for sure however without more information about the loading of the aircraft whether it was legitimate or not. If the flight attendant made a general announcement and it was the pursuer who did so, it is much more likely that it was real, because that is who the captain would have given the instruction to. If it was some random attendant just telling you personally not to move, it was probably BS.
That being said, moving around when a plane is taking off is a very bad idea at multiple levels because you could hurt yourself or someone else. You should always remain seated and buckled in when an aircraft is taking off or landing.